Advanced Welding Processes: Technologies and Process Control

High-energy density processes 147
Heating of the YAG rod will occur whilst the laser is running and some
form of water cooling is necessary to maintain reliable operation at the high
process powers required for welding. The overall process efficiency of the
Nd:YAG laser is less than 4% and large water chillers are again required.
The range of beam powers available with Nd:YAG systems is usually
lower than those achieved with CO 2 lasers and for welding 0.1–1 kW average
power devices are normally used. The YAG laser can, however, be pulsed to
very high peak power levels (i.e. up to 100 kW).
8.3.3 Laser beam characteristics
The CO 2 laser systems give good beam quality, which approximates to a
Gaussian distribution and a spot size of 0.1 mm. Higher power CO 2 systems
(above 1.5 kW) have multimode outputs which do not focus to such a small
spot size. Fibre-delivered Nd:YAG lasers have an integrated mode structure
due to total internal reflection in the fibre; this produces a uniform power
density across the beam diameter, which may be focused down to 0.2 to
1.0 mm. The literature on lasers often describes beam mode in terms of a
TEM (transverse electromagnetic mode) number. A Gaussian distribution
has a value of TEM 00 (Fig. 8.9). The difficulty of defining the outer edge of
the Gaussian distribution has led to the convention of measuring the beam
diameter in terms of the distance across the centre of the beam in which the
irradiance equals 1/e 2 (0.135) of the maximum irradiance; the area of a circle
of this diameter will contain 86.5% of the total beam energy. More complex
doughnut-shaped energy distributions may occur or a combination of beam
profiles may be produced in multimode operation where increased output
power is required at the expense of beam coherence. In practice, if the mode
(a)
Beam intensity
8.9 Laser beam intensity distribution: (a) TEM oo mode; (b) low-order
mode.
(b)